The present invention relates to a pressure sensor for detecting a pressure of fluid, particularly to a pressure sensor used to control a car engine and the like which is made using a semiconductor fine processing technology.
For example, Japanese Non-Examined Patent Publication No.7-7162 (1995) discloses a conventional pressure sensor.
The conventional pressure sensor is a capacitor type pressure sensor formed on a semiconductor substrate, comprising a reference capacitor which remains constant independently of a surrounding pressure and a sensing capacitance which varies as the surrounding pressure varies. Each capacitor consists of a first electrode which is a diffusion layer formed directly on a semiconductor substrate and a second electrode of a flexible diaphragm which is formed opposite to said first electrode with a cavity between said first and second electrodes and contains a conductive area made of single-crystal silicone. The cavity is sealed to keep a preset pressure. Both reference and sensing capacities use a diffusion layer formed on the semiconductor substrate as the first electrode. The pressure sensor disclosed by Japanese Non-Examined Patent Publication No.7-7162 (1995) is characterized in that the flexible diaphragm moves as the surrounding pressure varies and thus the capacitance between the first and second electrodes varies.
For higher accuracy, a well-known conventional capacitor type pressure sensor employs a technology comprising the steps of
using a reference electrode which has a capacitance almost equal to that of an active capacitor and will not vary substantially as means to cancel a characteristic change of the active capacitor due to external disturbing factors such as noises and uneven processing which is not related to the pressure change and
calculating a capacitance difference between the reference capacitor and the active capacitor or a ratio thereof by a detecting circuit.
For higher accuracy of a capacitor type pressure sensor comprising a reference capacitor, an active capacitor and a detecting circuit on a semiconductor substrate, there may arise a problem that an unwanted parasitic capacitance (or a junction capacitance) which may vary due to the concentration of impurities in the substrate or a potential difference between the substrate and the reference capacitor electrode generates between the reference capacitor electrode and the semiconductor substrate. Therefore, when the semiconductor substrate is grounded or connected to a detecting circuit which is used as a power supply, the parasitic capacitance between the electrode and the semiconductor substrate varies greatly relative to a preset capacitance formed between reference capacitor electrodes and the SN ratio of the reference capacitor including the parasitic capacitance relative to the change of the active capacitor increases and varies. Consequently the accuracy of measurement of fluid pressure will go down.
For a capacitor type pressure sensor comprising, on a semiconductor substrate, an active capacitor which varies as the surrounding pressure varies, a reference capacitor which does not vary substantially as the surrounding pressure varies, and a circuit which is electrically connected to said active capacitor and to said reference capacitor, detects a capacity difference between said capacitors or a rate thereof, and works using the potential of the semiconductor substrate, the aforesaid problem can be solved by forming a reference capacitor electrode on the semiconductor substrate through a dielectric.